Police said a man videotaped under her skirt as the train was rolling through Kew gardens. How she spotted the camera is still not clear.
But she told police it was sitting face up in a gym bag, angled to videotape beneath her skirt.
The girl flagged down a NYPD transit officer, who seized the camera and arrested the man who owned it.
The suspect was identified as Jose Gil, 39, of Queens.
He is charged with unlawful surveillance on a minor.